How to Make It
For blueberries: Combine all ingredients except berries and shallot in a medium saucepan. Bring to simmering over medium heat, then reduce heat and simmer 5 minutes. Stir in berries and cook until all of them darken, about 2 minutes. Stir in shallot and let stand at room temperature about 2 hours.
Meanwhile, prepare chicken: In a large bowl, combine lemon zest, coriander, salt, and pepper. Rub evenly over chicken. Let stand at room temperature about 1 hour. Trim ends from watercress (to leave long sprigs) and rinse. Wrap in a kitchen towel and chill to crisp.
Heat a grill to medium (400° to 450°). Rub chicken with oil, then grill it, turning once, until no longer pink in thickest part, 20 to 30 minutes.
Arrange chicken on a platter or plates with watercress. Pour blueberries into a bowl and serve to spoon over chicken and greens.
Make ahead: Blueberries, up to 1 week, chilled.